The present invention relates to a fiberoptic spectral filter for multiplexing and demultiplexing signals on a fiberoptic waveguide.
The maximum information capacity of a single mode fiberoptic waveguide is limited by the spectral line width of the source and the speed of the detectors that receive the optical signal. Highly monochromatic light sources, such as lasers, are available that produce signals of very narrow spectral width. The electronics needed to process the signals, however, are limited in their ability to interpret the signals sent over a single mode fiberoptic cable. Fiberoptic communications systems thus cannot transmit all the information that a given fiberoptic cable is capable of carrying.
The amount of information that a fiberoptic communications system can carry can be increased by multiplexing the transmitted signals. Wavelength division multiplexing (WDM) allows one fiber to transmit several different optical channels. Each channel comprises light having a different wavelength, i.e. different color or different frequency. Techniques are known for launching different channels into the fiberoptic cable (multiplexing) and for separating the channels (demultiplexing). Demultiplexing requires one or more spectral filters to separate the different wavelengths.
Several techniques have been proposed for performing the wavelength division needed for demultiplexing. These techniques often involve using grating reflectors, dichroic filters and fused taper couplers. The grating reflectors and dichroic light filters require that the light leave the fiber to be multiplexed and subsequently returned to the fiber. This process results in a loss of signal power and may, in addition, require critical mechanical alignments that are complex and expensive to make.
A fused taper coupler is a WDM device that is made entirely of fiberoptic material. It can perform the multiplexing in a single mode fiberoptic system with very low insertion loss. Such a device has no need for complex mechanical alignments. A fiberoptic WDM device, however, must attain a high rejection of cross talk from neighboring channels.
Wavelength selective fiberoptic couplers may be made from identical or from dissimilar fibers. Light is transferred between the cores of the two fibers through their evanescent fields along an interaction region. The power transfer is strongly influenced by the propagation constant of the dominant modes of the fibers. The fibers are phase matched when the modes have equal propagation constants such that maximum power transfer occurs between the fiber cores. If the fibers are identical, however, their dominant modes are phase matched at all frequencies such that a complete transfer of power is possible at any frequency. The transfer is determined entirely by the coupling links of the fibers. Thus, such a coupling does not exhibit high frequency selectivity.
Fiberoptic couplers made from dissimilar fibers, such as shown in Gordon U.S. Pat. No. 4,673,270, can have a propagation constant in the dominant modes that is equal at only discrete frequencies. These frequencies correspond to the intersection points of the dispersion characteristics of the modes. The couplers can be phase matched at the intersection frequencies so as to create more sharply defined transmission curve. Fiberoptic couplers made from dissimilar fibers are therefor preferred as spectral filters.
Monochromatic light sources, such as lasers, emit light over a very narrow spectral bandwidth. It is possible to adjust or "tune" the frequency at which the laser emits light. Current lasers are limited in the range of wavelengths over which they can be tuned. Thus, the number of wavelengths at which light can be generated is inherently limited. Furthermore, fiberoptic bundles can transmit light efficiently only over certain wavelength ranges. It is thus desirable to select lasers that emit light over a narrow range of frequencies that correspond to the maximum transmission efficiency of a given fiberoptic material.
The constraints of both transmission efficiency and the limits imposed by tuning the laser light source place a premium on being able to transmit as many channels of information as possible over the most narrow frequency range possible. The ability to multiplex a number of channels over a narrow frequency range requires the ability to distinguish between channels that have very similar wavelengths. Laser light source can generally be tuned to the appropriate frequency. The major limit, however, is the ability to distinguish between these light sources at the demultiplexing end of the fiberoptic cable.
The ability to filter the various spectral components of a fiberoptic signal can be determined by adjusting a variety of the parameters in a fiberoptic coupler. For example, the difference the index of refraction of the cores of the fiberoptics, their separation, and the index of refraction of the surrounding cladding material can all be varied. The optimization of these various parameters, however, is a complex task that is not easily solved.